This invention relates to a method of coloring a glass sheet, the method being of the type commonly called a staining method in which silver ions are diffused into a surface layer of the glass sheet and caused to undergo reduction in that surface layer.
A broad concept of coloring a glass sheet by diffusing silver ions into a surface layer of the glass sheet and causing the diffused silver ions to undergo reduction has already been embodied in some methods. According to Japanese Patent Applications Primary Publication Nos. 48(1973)-43714 and 48(1973)-43715, equivalent to Canadian Pat. Nos. 1,006,700 and 1,006,699 both issued Mar. 15, 1977, for example, a silver salt such as silver sulfate, silver nitrate or silver chloride, in the form of paste, is applied to a surface of a glass sheet, the glass sheet containing ions, reducing such as tin ions, which have been diffused to a small depth in the glass surface. The paste is heated on the glass surface to cause diffusion of silver ions into the surface layer of the glass sheet. The diffused silver ions are reduced by the reducing ions in the glass thereby producing color. As a modification, Japanese Patent Application Primary Publication No. 49(1974)-14510, published Feb. 8, 1974, shows a coloring method having the steps of applying a mixture of a silver compound, such as silver sulfide or silver chloride, and a glass frit to a surface of a glass sheet, heating the glass sheet and the applied mixture to cause diffusion of silver ions into a surface layer of the glass sheet, removing the residue of the mixture from the glass surface, and then heating the glass sheet in a reducing gas atmosphere to cause the diffused silver ions to undergo reduction and produce color.
In practice, however, these known staining methods encounter various problems and inconveniences that are mostly attributed to the chemical and physical properties of the silver compound. Firstly, silver compounds used in these methods are generally sensitive to light and liable to decompose to grayish or blackish substances. Secondly, these silver compounds tend to deliquesce by the effect of moisture or react with other ingredients of the silver compound paste and, hence, it is difficult to prepare the paste. Thirdly, these silver compounds are rather low in chemical stability, so that sometimes a glass sheet is colored nonuniformly because of thermal decomposition of the silver compound heated on the glass surface. Furthermore, when heated, these silver compounds emit corrosive decomposition gases which corrode the colored surface of the glass sheet and deteriorate the transparency of the colored glass sheet and exert a significantly detrimental effect upon the furnace used in the heating process.